Heretofore, a white-light emitting device constructed by combining a gallium nitride (GaN) base light-emitting diode (LED) as semiconductor light emitting device and phosphors as wavelength converting material has been noted as light source for a display or lighting system, making use of its advantages of low power consumption and long operating life.
As a typical light emitting device, a white LED comprising In-added GaN base blue LED and Ce-activated yttrium aluminium garnet base yellow phosphor can be cited particularly. However, there are problems, as already pointed, that the light amount in the range of red (600 nm or longer) and blue green (480 nm to 510 nm) is small, and that the general color rendering index Ra of the light from the light emitting device is low. Therefore, an improvement has been demanded.
To solve this problem, in Patent Document 1, it is disclosed that white LED emitting white composite light can be obtained by exciting, using blue LED, phosphor consisting of a red phosphor such as (Ca1-a-bSraEub)S:Eu2+, which is used for increasing red light component in addition to the light component of a green phosphor such as (Y1-a-bGdaCeb)3(AL1-cGac)5O12. It shows, in addition, a method of obtaining white light by means of adjusting the weight ratio of the green phosphor to be 40% to 80% of a mixture of the green and the red phosphors. The red phosphor used here is a substance which can be excited by the light emitted from a green phosphor. However, as the luminous efficiency of the red phosphor is lower than that of the green phosphor, when the mixture having the above-mentioned combination and weight ratio of these phosphors is used, the weight ratio of the red phosphor is necessary to be 20% to 60%, which is relatively much. This then leads to a problem of reduction in luminous flux emitted from the white LED because green light emitted from the green phosphor is absorbed by the large amount of the red phosphor, having low luminous efficiency. Moreover, as the red phosphor used is low moisture-resistant, sulfide base red phosphor, there are problems of ease of deterioration, and of high production cost due to the difficulty in synthesis. This leads to the problems of the white LED, which is obtained by using the red phosphor, of low durability and high production cost. Furthermore, as the color of the emitted light from the green phosphor used is a little too yellowish, there is another problem that the shortage of blue-green range of emitted light causes inferior color rendering. In non-Patent Document 1, white LED, using SrGa2S4:Eu2+ as green phosphor and ZnCdS:Ag,Cl as red phosphor, is disclosed. There are problems, with this LED too, of insufficient luminous flux, insufficient color rendering and vulnerability of the sulfide to deterioration when the white LED is used.
[Patent Document 1] Japanese Patent Laid-Open Publication (Kokai) No. 2003-243715
[Non-Patent Document 1] J. Electrochem. Soc. Vol. 150 (2003) pp. H57-H60